The present invention generally relates to packaging and more particularly to an apparatus and method for making unitary packages which hold a plurality of components, each package containing a load wrapped in a web of stretched film.
Case packing or boxing is a common way of shipping multiple unit products. The multiple unit products are generally stacked in a corrugated box or are wrapped with kraft paper with the ends of the kraft paper being glued or taped. Another way of shipping such products is by putting a sleeve or covering of heat shrinkable film around the products and shrinking the sleeve to form a unitized package. The use of heat shrinkable film is described in U.S. Pat. Nos. 3,793,798; 3,626,645; 3,590,509; and 3,514,920. A discussion of this art is set forth in U.S. Pat. No. 3,867,806.
Another common method of wrapping loads is with rotary stretch wrapping machines. These rotary machines are commonly referred to as spiral or full-web machines, and can operate with the load rotating to pull stretched film web around it. Alternatively, the load can be stationary and stretched film wrapped around the load with a rotating film dispenser.
A typical state-of-the-art full-web apparatus is disclosed in U.S. Pat No. 3,867,806.
The use of spiral wrapping machinery is well known in the art and representative machines are typified by U.S. Pat. Nos. 3,003,297; 3,788,199; 3,683,425; and 4,136,501.
Additional references of interest which are pertinent to rotatable drives for wrapping packages are disclosed in U.S. Pat. Nos. 3,820,451; 3,331,312; 3,324,789; 3,309,839; 3,207,060; 2,743,562; 2,630,751; 2,330,629; 2,054,603 and 2,124,770.
The film stretching means on all currently marketed pallet stretch wrapping devices employ either direct or indirect friction to restrict the film as it is being wound onto the load during the wrapping process. The restriction is either applied to the roll of film itself (direct friction) or applied to the film after it is unwound from the film roll (indirect friction). The pallet and load serve as the winding mandrel providing all of the pulling force required to elongate the film.
The earliest type of stretch wrapper utilized a direct friction device in the form of a brake that is connected to the film roll through the core as shown in FIG. 1. The torque from the frictional brake device acted on the center of the film roll and as the roll changed diameter, the voltage to the brake was altered, either by the operator or automatically by a sensing device. A later film roll brake device, illustrated by U.S. Pat. No. 4,077,179, and FIG. 2 herein, utilizes a frictional brake attached to a shaft with a roller which is pressed against the freely mounted film roll. The film roll brake eliminates the need to change the brake force during the consumption of the film roll.
Various prior art indirect friction film stretching devices have been employed to restrict the film as it is wound onto the pallet during the wrapping process. One of these devices, commonly referred to as an "S" type roller device, is shown in FIG. 3, and utilizes an idle roller followed by a braked roller over which the film is threaded prior to wrapping the load. The function of the two rollers is to align the film for maximum contact with the braked roller. Another indirect friction device having fixed bars was marketed by Radient Engineering Corporation under the trade name POS-A-TENSIONER and has been subsequently marketed by the Kaufman Company under the trade name TNT. This device, shown in FIG. 4, has a series of fixed, non-rotating bars positioned adjacent to the film roll. The film web is threaded around the bars whose relative angles can be changed for ultimate tensioning. As the film web is attached to the pallet it is drawn across the bars and the friction between the film and the smooth surface of the bars provides a restriction causing the film to stretch. This device uses multiple bars with the film web stretching incrementally between each bar. Neck down of the film web increases between each bar and the load bears the force. As the load rotates, the wrap angle changes from the last bar so that the wrapping force greatly varies depending on the relative angles. The frictional restraint is determined by the vector of the film web on each bar. Thus, the device is very sensitive to the force placed on the unwind roll and the force increases as the roll size decreases adding additional force on the system. Furthermore, there must be some friction placed on the supply roll to prevent backlash. While this device solves to some degree the irregularities of the brake and the hostility of the film roll, it can only apply limited stretch to the load and does not handle different film compositions with any degree of standardization.
Another stretch wrapper device was introduced by the Anderson Company at the PMMI Show in Chicago in 1978. This device interconnected the turntable drive motor with a pair of nip rollers immediately downstream from the film unwind roll, as shown in FIG. 5. The nip rollers were synchronously driven with the turntable rotation through a variable transmission which could be increased or decreased in speed relative to the turntable rotation speed. Thus the stretch on the film was affected between the constant-speed nip rollers and the pallet turning. It is not known if this machine was ever commercialized, principally because of its inability to achieve satisfactory stretch over the load corners due to its failure to respond to the speed change that these corners represented. The pallet, as the film accumulating mandrel, provided the total force that was required to stretch the film from the driven nip rollers with all of the stretch occuring after the passage of the single pair of nip rollers to the pallet.
In addition to the previously noted prior art, direct friction pallet stretch wrapping machines of the pass through type have been manufactured by Weldotron and Arenco (Model No. MIPAC). These machines have a significant problem in stretching the film and normally stretch film around the load in the range of about five to ten percent. These machines depend on being able to drive the pallet and associated load through a stretched curtain of film to place the stretching force on the front or sides of the load.
Since most pallet loads will not hold together while being subjected to these unequal forces, the film web is normally tensioned after the film seal jaws begin their inward travel over the end of the pallet load. This form of tensioning severely limits the degree of elongation of film which is able to be achieved and pulls excess film around the two rear corners of the load while the jaws are closing. Thie frequently causes film tears when the film is stretched more than ten percent.
When low stretch rates of one to ten percent are produced, several packaging problems occur. The unitizing containment forces on the load are less than the optimum force which can be obtained. This minimizing containment forces can result in a potential loosening of the film wrap during shipment where the load settles and moves together thereby reducing the girth.
Another pass through machine described by French Pat. No. 2,281,275 assigned to SAT discloses the prestretching of plastic film by taking the film web from the film roll through a powered roller system having a speed differential of V.sub.2 -V.sub.1 which stretches the film. The film leaving the second set of rollers is drawn off at a speed which is equal to or less than V.sub.2 as it is wrapped around the load. V, which is the speed of rotation of the pallet load, is less than or equal to V.sub.2, the speed of the stretched film coming off of the second roller assembly.
Although the French Patent appears to achieve film web stretch in excess of the one to ten percent range obtained in the aforementioned pass through stretch wrapping machines, other problems remain. The system requires manual operation or complex automatic feedback to accomodate the change in film take-up speed as the pallet load surfaces pass by the downstream rollers. This reference does not teach the benefit of stretching the film above the yield point with increased strength per cross-sectional area and increase in modulus. There is furthermore no teaching of reducing the force on the portion of the film web between the downstream powered rollers and the load with inelastic strain recovery as a technique for reducing wrapping force while holding high levels of elongation.
A commercial model based on FIG. 8 of the '275 reference is currently being marketed by SAT. In this embodiment the film web is pre-stretched by extending a pair of rollers forward while braking the film rolls. The load is carried into the pre-stretched "U" shaped sleeve and the rollers are transported back of the load allowing the sleeve to engage the load. Sealer bars are then projected inward to seal the web ends together.
The aforementioned stretching devices do not maintain a consistent force in stretching the film web. These brake devices are subject to variation due to their physical construction and their sensitivity to speed change caused by passage of corners of the load and the resultant sudden speed-up and slow-down of film drawn from the feed roll.
The elasticity of the stretched plastic film holds the products of the load under more tension than either the shrink wrap or the kraft wrap, particularly with products which settle when packaged. The effectiveness of stretch plastic film in holding a load together is a function of the containment or stretch force being placed on the load and the ultimate strength of the total layered film wrap. These two functions are determined by the modulus or hardness of the film after stretch has taken place and the ultimate strength of the film after application. Containment force is currently achieved by maximizing elongation until just below a critical point where breaking of the film occurs. Virtually all stretch films on the market today including products of Mobil Chemical Company (Mobil-X, Mobil-C and Mobil-H), Borden Resinite Division PS-26, Consolidated Thermoplastics, Presto, PPD and others are consistently stretched less than thirty percent in most commercial applications despite a manufacturer's laboratory rated capacity in excess of three hundred percent in most cases.
This problem of obtaining less stretch on commercial wrapping than that available under laboratory conditions centers on several facts. A square or rectangular pallet which is typically positioned off of its center of rotation is used as the wind up mandrel for the purpose of stretching film. A typical 40".times.48" pallet positioned 3 to 4 inches off of its center of rotation will experience a speed change of up to 60% within one quarter revolution of the turntable.
In this regard, FIGS. 16 through 21 illustrate the manner in which constant rotation of a palletized load placed slightly off center on a turntable will result in significant variations in tension on the film web being wrapped around the load. Since the turntable rotates at a constant angular speed, the film web is drawn to and around the rotating pallet load at a speed which is determined by the distance between the axis of rotation and the point at which the web contacts the load. The axis of rotation in each of the illustrations is at point A and the distance between the axis and the film contact point is illustrated by an arrow B. The axis of rotation A is offset slightly from the true geometric axis C of the palletized load. It can thus be seen that as the load is rotated in a clockwise direction, the distance from the axis of rotation A to the film web contact point remains constant in FIGS. 16 through 19. However, as the load is rotated from the position shown in FIG. 19 to its position shown in FIG. 20, the distance between the axis of rotation and the film contact point increases markedly, thus increasing the speed at which the film is drawn from the roller and the force between the downstream stretch roller and the pallet load.
In addition to the off centering problem most pallet loads are irregular in shape with vertical profiles which produce a significant puncture hazard to highly stretched film being wound around them. Further, some unit loads are very susceptible to crushing forces of the stretched film. Because of pallet load changes and inconsistencies within the film roll, the operator typically continues to reduce the tension settings until there are no failures. Thus the inconsistencies of films, stretching devices, and pallet loads produce an environment where very few stretch films are actually stretched to their optimum yield.
The major problems with current stretch technology are that stretch is produced by frictional force devices to restrict the film travel between two relatively hostile bodies. On the one hand the film roll is subject to edge wandering and feathering, while on the other hand the rotating pallet with its irregular edges and rapidly changing wind-up speeds severely limits the level of elongation achieved. The ultimate holding forces of the film cannot be brought to bear on the load because the film cannot be stretched enough. Even if the film could be stretched enough the high wrapping forces can disrupt or crush many unit loads. The use of high modulus films, such as oriented films, does not produce the yield benefits of the current invention, since these higher modulus films would have to be significantly stretched in order to achieve the rubberband effect and moldability required for irregular loads.
It therefore can be understood, since the pallet provides the forces for stretching the film, that stretch percentages achieved on the pallet and the stretch force achieved are intertwined in all prior art devices. As previously indicated, high stretch percentages are required to achieve the benefits of high yield but the high stretch forces incurred at these high stretch percentages cause premature film rupture and potential crushing of the load.
In an attempt to solve the aforementioned problems several other devices have been developed by the present inventors.
One device called the powered stretch embodiment stretches the film web above its yield point between two sets of powered rollers prior to transporting the film to the pallet, increasing its modulus while reducing its cross-sectional area.
Since the film stretches between the rollers, all stretching action is isolated from the roll and the pallet. It also removes the dependence of the stretch force and elongation level. While the device can be used to wrap light or crushable loads it has several problems in actual use. The controls necessary to compensate for the interacting speed changes are very complex and prohibitively expensive. Thus, the device generally will require feedback controls to sense force change and maintain the force level.
Another known device manufactured by Lantech Inc., under the trademark "ROLLER STRETCH" utilizes the film web to drive the apparatus. This device addresses several of the aforementioned problems. Since the film is pre-stretched between the rollers, it isolates the stretching action from between the film roll and the pallet. This device provides a consistent level of stretch and, most importantly, responds to force and speed changes without complex feedback controls as can readily be seen in the graph of FIG. 21. A problem inherent with the ROLLER STRETCH device is that it has a dependence between the percentage of stretch that can be achieved and the stretch force that will be required to elongate the film. It should be noted that although these two factors are connected, the film web drive device significantly lowers the stretch force for a given elongation level, at or below the balance point as is seen by FIG. 22. This is due to the mechanical advantage between the film driven rollers.
Balance is achieved when elongation between the rollers (E.sub.1) is equal to elongation on the load (E.sub.2). The relatively higher forces between the closely spaced rollers are overcome by the lower force required to drive the device by the film between the roll and the load. The stress/strain curve experienced between closely spaced rollers is substantially higher than the curve where film is allowed to expend the pulling force. Thus, the film to the load affects this higher force between the rollers aided by the mechanical advantage of the differential pulley relationship of the gear connected rollers. At balance point 414 the elongation on the load (E.sub.2) equals elongation between the rollers (E.sub.1) and the mechanical advantage represents the differences between the forces corrected for friction. This limits the film drive device to an elongation level on most films of under 120 percent elongation between the rollers.
It is therefore apparent that there exists a need for a pallet load wrapping apparatus which reduces or eliminates the dependence between the percentage of stretch that can be achieved and the force that will be required to drive the pre-stretch device.